Testing alternative hypotheses of Neotrigonia (Bivalvia: Trigonioida) phylogenetic relationships using cytochrome C oxidase subunit I DNA sequences

Uncertainties regarding the phylogenetic history within the Bivalvia have impeded attempts to understand evolution within the group. Estimating the evolutionary relationships surrounding the Trigonioida has been especially problematic and has led to disparate hypotheses regarding (1) the origin and subsequent diversification of unionoid bivalves and (2) autobranch gill character state transitions. In order to test alternative hypotheses of trigonioid phylogeny, 613 base pairs of DNA sequence of the cytochrome c oxidase subunit I gene were analyzed from 14 species of bivalves (ingroup) and three species of non-bivalve mollusks (outgroup). All phylogenetic analyses, using either nucleotide or inferred amino acid sequences, produced trees that robustly placed Neotrigonia (Trigonioida) as the sister taxon to a monophyletic Unionoida. Furthermore, the Autobranchia, Veneroida, and Mytiloida were supported as monophyletic groups in these analyses, whereas the Bivalvia was not. These phylogenetic relationships suggest that (1) there was a single invasion of freshwater by a unionoid bivalve ancestor, (2) trigonioid rather than veneroid bivalves gave rise to the Unionoida, (3) either the eulamellibranchous or filibranchous gill condition has evolved multiple times within the Autobranchia, and (4) the molluscan bivalved body plan may have evolved more frequently than traditional phylogenetic hypotheses suggest.